System and method of preventing damage to metal traces of flexible printed circuits

ABSTRACT

Stiffeners in are provided in a flexible printed circuit to prevent damages to leads and traces of the flexible circuit caused by bending, folding and other stresses.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns flexible printed circuits.

2. Description of the Related Art

Flexible printed circuits (FPCs) are electronic circuits that arecommonly used in a variety of modern electronic devices. A flexibleprinted circuit has circuit traces and possibly electronic componentsdeposited onto a flexible substrate. Flexible printed circuits typicallycomprise plastic substrates and etched thin metal foils and are so namedbecause of their ability to bend or flex. They have the advantage ofbeing thin, thus saving space, and of being easily moldable to the shapeof the electronic device. They are often used to form a connectionbetween two separate circuits.

SUMMARY OF THE INVENTION

In one embodiment, the invention comprises an electronic systemcomprising a panel and a flexible circuit connected to the panel. Theflexible circuit comprises at least one stiffener, wherein the at leastone stiffener resides in either one or both of a region where theflexible circuit crosses an edge of the panel such that the stiffenerextends across the edge; and a region where the flexible circuit bendssuch that the stiffener extends around the bend. In another embodiment,an electronic system comprises a panel, a flexible circuit, and a meansfor stiffening a portion of the flexible circuit. Another embodimentcomprises a method for preventing damage to metal conductors on or in aflexible circuit. The method comprises providing at least one stiffenerat either one or both of a region where the flexible circuit overlaps apanel edge, and a region where the flexible circuit is bent.

BRIEF DESCRIPTION OF THE FIGURES

Exemplary embodiments of the invention are explained in greater detailin the following description and are illustrated in the drawings, inwhich:

FIGS. 1 a-c show a cross-sectional view of a flexible circuit attachedto a display panel;

FIG. 2 shows a close up view of the micro-cracks that appear in themetal races of flexible circuits;

FIG. 3 shows a cross-sectional view of a flexible circuit according toone embodiment of the invention;

FIG. 4 shows a top view of the flexible circuit of FIG. 3;

FIG. 5 shows a top view of a flexible circuit according to anotherembodiment of the invention;

FIG. 6 shows a top view of a flexible circuit according to a certainembodiment of the invention;

FIG. 7 shows a top view of a flexible circuit according to oneembodiment of the invention; and

FIG. 8 shows a top view of a flexible circuit according to oneembodiment of the invention.

DETAILED DESCRIPTION

Flexible circuits are often used to form a connection between separatecomponents of a device. For example, a flexible circuit may be used toconnect a power module to another component used by an electronicdevice. The inventors manufactured a display for use in an electronicdevice. The display used a flexible circuit to connect a display arrayto a module that provides power to the display and provides controlsignals to control the images displayed by the display array. Theflexible circuit was connected to the display array and the controlcomponents during the assembly process.

FIG. 1A represents a side cross-section view of an example of a flexiblecircuit and panel that was used in an electronic device. The flexiblecircuit 3 is connected to a panel 2. The panel 2 may comprise a displayarray including but not limited to LCD displays ormicro-electromechanical system (MEMS) displays. The flexible circuit isbent at bend region 6 and is connected to the other components of theelectronic device through connector 4.

FIG. 1B represents a side cross-section view of another example of aflexible circuit and panel that was used in an electronic device.Flexible circuit 3 is connected to panels 2 and 9. Panels 2 and 9 haveglass covers 5 and 7. Components 12 are placed above the region wherethe flexible circuit 3 connects to panels 2 and 9. The flexible circuit3 is bent at bend region 6.

FIG. 1C represents a side cross-section view of another example of aflexible circuit and panel that was used in an electronic device.Flexible circuit 3 is connected to panels 2 and 9. Panels 2 and 9 haveglass covers 5 and 7. The flexible circuit is bent at bend region 6 anda component 12 is located within bend region 6.

However, after construction electronic devices in this manner, it wasdiscovered that there was a high incidence of failure in the displayarrays of the electronic device. Due to the high failure rate,manufacturing costs of the electronic device were increased.

After careful analysis as to the cause of the display array failures,the inventors discovered microscopic cracks in the metal traces that ranthrough the flexible circuit which connected the display arrays to thecontrol and power module. FIG. 2 shows a close up view of themicro-cracks that occur in the metal traces of the flexible circuit.Metal trace 8 runs through the enlarged section of the flexible circuitand micro-crack 10 has occurred in metal trace 8. Upon further analysis,it was discovered that the microscopic cracks were induced during theassembly and test process. In the assembly and test process the flexiblecircuit was often bent, twisted or curved in order to position itcorrectly with respect to the rest of the electronic device. If theflexible circuit was bent or twisted too much, microscopic cracks wouldoccur in the metal traces of the flexible circuit. Such bending,twisting or curving of the flexible circuit is often unavoidable duringthe manufacturing and testing of the electronic device. Furthermore, thetraces may be weakened at the bend region when a permanent bend isrequired by subsequent display module integration and assembly. It wouldbe preferred if the stresses resulting from the bending, twisting ordeviating the flexible circuit from a flat linear profile could bereduced to the point where the stresses would no longer causemicro-cracks in the metal traces.

FIG. 3 represents a side cross-section view of a flexible circuit andpanel according to an embodiment of the invention. Flexible circuit 15is connected to panel 12. Panel 12 may comprise a display array.Flexible circuit 15 is bent at a bend region 13. One or more stiffeners14 are placed at the bend region of flexible circuit 22 as is also shownin FIGS. 4-8, and at the region where the flexible circuit overlapspanel 12. A “bend region” refers to any region where a flexible circuitis curved, bent, twisted or otherwise deviates from a flat linearprofile. The bend region may occur anywhere in the flexible circuit andis generally defined by the display module manufacturing process usingthe flexible circuit. FIG. 3 is a cross section view of the flexiblecircuit 15, and only shows the placement of stiffeners 14 in the crosssection view. As FIG. 3 shows, the stiffener 14 overlaps with the edgeof the display panel 12. There may be multiple stiffeners 14 placedalong the edges or within the central region of the flexible circuit 15.FIGS. 4, 5, 6, 7, and 8 show example placements of the stiffeners 14.The flexible circuit 15 is connected to the other components of theelectronic device through connector 16.

FIG. 4 schematically shows a top view of the flexible circuit of FIG. 3.Flexible circuit 15 is connected to panel 12. Panel 12 may comprise adisplay array. Stiffeners 14 reside in the region where the flexiblecircuit crosses an edge of panel 12 such that the stiffeners extendacross the edge of panel 12 and such that they are along the edge offlexible circuit 15.

FIG. 5 schematically represents the flexible circuit and panel accordingto another embodiment of the invention. Flexible circuit 17 is connectedto panel 19. Stiffeners 18 reside in the region where the flexiblecircuit crosses the edge of panel 19 such that the stiffeners may extendacross the edge of panel 19, or may be very near the edge of panel 19.Stiffeners 18 are on or near the edge of flexible circuit 17. Stiffener20 resides along an inner portion of the flexible circuit 17, and may becombined with the flexible circuit metal traces. Please note that thereare multiple locations where the stiffeners 18 and 20 may reside andthese locations are not limited to what is shown in FIG. 5. For example,an embodiment of the invention may not place any stiffeners 20 in thecentral region of the flexible circuit 17 and only place stiffeners 18along the edges of the flexible circuit 17. Another embodiment may notplace any stiffeners 18 along the edges of flexible circuit 17 but onlyplace one or more stiffeners 20 along a central portion of flexiblecircuit 17.

FIG. 6 shows one embodiment of a flexible circuit 26 according toanother embodiment of the invention. Stiffeners 24 reside along the edgeof flexible circuit 26. Notches 30 are cut into the flexible circuit 26to define a bend region 32 for flexible circuit 26. The flexible circuitmay be bent at the bend region as shown in FIG. 3. Flexible circuit 26is connected to other components of the electronic device throughconnector 36. Additionally, the flexible circuit 26 has mounts 38 forpassive components including but not limited to resistors andcapacitors.

FIG. 7 shows another embodiment of a flexible circuit 15 according toanother embodiment of the invention. Flexible circuit 15 is connected topanel 12. Stiffeners 34 reside near the edges of flexible circuit 15.Stiffeners 34 are continuous and have three rectangular sections thatare connected by necks 38. The necks 38 may define one or more bendregions 39 for the flexible circuit 15.

FIG. 8 shows another embodiment of a flexible circuit 15 according toanother embodiment of the invention. Flexible circuit 15 is connected topanel 12. Stiffeners 40, 50 and 60 reside near the edges flexiblecircuit 15. Stiffeners 40, 50 and 60 are discrete and are not connectedto each other. In addition, the gaps between stiffeners 40, 50 and 60may define one or more bend regions 39 for the flexible circuit 15.

Stiffeners 14, 18, 20, 24, 34, 40, 50, and 60 may be connected togrounding planes, connected to power sources, connected to test traces,or connected to signal sources.

The stiffeners 14, 18, 20, 24, 34, 40, 50, and 60 will stiffen theflexible circuits 15, 17, and 26 in the regions where they are placed.The stiffeners 14, 18, 20 and 24 will absorb the strenuous bendingstresses and increase the flexural strength, tensile strength andelasticity of the flexible circuit. This stiffening of the flexiblecircuits 15, 17, and 26 helps prevent damages to traces 22 and 33 as itprevents the flexible circuits 15, 17, and 26 from being bent or twistedto the point where it damages traces 22 and 33. Bending of a flexiblecircuit may occur in a variety of stages during the construction ofelectronic devices that use flexible circuits, for example, installationand testing stages.

Stiffeners 14, 18, 20, 24, 34, 40, 50, and 60 in FIGS. 3, 4, 5, 6, 7,and 8 may be any shape or form. For example, the stiffeners may berectangular, circular, oval shaped or triangular. The invention is notin any way limited to a particular shape or form of the stiffeners. Inaddition, stiffeners 14, 18, 20, 24, 34, 40, 50, and 60 in FIGS. 3, 4,5, 6, 7, and 8 may be continuous or may be divided into discreteportions as shown in FIGS. 7 and 8 respectively.

Stiffeners 14, 18, 20, 24, 34, 40, 50, and 60 in FIGS. 3, 4, 5, 6, 7,and 8 may be created during the manufacturing process of the flexiblecircuits 15, 17, and 26. In general, flexible circuits are manufacturedby depositing a layer of conductive material onto a flexible substrateor with copper foil on a base film. The conductive layer or the copperfoil is then subjected to an etching process which etches away theportions of the conductive layer that may or may not be used by thecircuit. The stiffeners 14, 18, 20 and 24 may be created during thisetching process. In conventional manufacturing methods, the entireexcess of the conductive layer was etched away. In the new method,strips of the conductive layer will not be etched in order to retainmetal strips to form the stiffeners. As will be understood by those ofskill in the art, plating on the stiffeners can be achieved to producethe same surface finish as on the rest of the traces if the plating bus44 is extended to the stiffeners and with a trace 42 attached to eachstiffener. This method of creating the stiffeners allows the stiffenersto be added to the flexible circuit at no additional cost and does notrequire any changes to the overall manufacturing method of the flexiblecircuit.

Please note that the invention is not limited to any of the aboveembodiments and may be applied to any electronic device that uses aflexible printed circuit.

1. An electronic system comprising: at least one panel; at least oneflexible circuit connected to the panel; the flexible circuit comprisingat least one stiffener, wherein the stiffener is coplanar with traces ofthe flexible circuit, wherein the stiffener is positioned along acentral portion of the flexible circuit and parallel to an edge of theflexible circuit, and wherein the at least one stiffener resides ineither one or both of: a region where the flexible circuit crosses anedge of the panel such that the stiffener extends across the edge; and aregion where the flexible circuit bends such that the stiffener extendsaround the bend.
 2. The electronic system of claim 1 wherein thestiffener is positioned along an edge of the flexible circuit. 3.(canceled)
 4. The electronic system of claim 1 wherein the panelcomprises a display array.
 5. The electronic system of claim 1 whereinthe flexible circuit comprises notches in the region where the stiffeneris present.
 6. The electronic system of claim 1 wherein the stiffenercomprises metal.
 7. The electronic system of claim 6 wherein thestiffener comprises copper.
 8. The electronic system of claim 6 whereinthe stiffener is grounded.
 9. The electronic system of claim 6 whereinthe stiffener is connected to a signal source.
 10. The electronic systemof claim 6 wherein the stiffener is connected to a power source.
 11. Theelectronic system of claim 6, wherein the stiffener is connected to atest trace.
 12. The electronic system of claim 1, wherein the stiffeneris rectangular in shape.
 13. The electronic system of claim 1, whereinthe stiffener is not rectangular in shape.
 14. The electronic system ofclaim 1, wherein a neck connects two sections of the stiffener to form aregion where the flexible circuit is bent.
 15. The electronic system ofclaim 6 wherein the stiffener is one millimeter to two millimeters inwidth.
 16. A electronic system comprising: at least one panel; at leastone flexible circuit; and means for stiffening a portion of the flexiblecircuit wherein the means for stiffening is co-planar with traces of theat least one flexible circuit and wherein the means for stiffening ispositioned along a central portion of the flexible circuit and parallelto an edge of the flexible circuit.
 17. The electronic system of claim16 wherein the panel comprises a display array.
 18. The electronicsystem of claim 16 wherein the flexible circuit comprises notches in theregion where the stiffening means is present.
 19. The electronic systemof claim 16 wherein the stiffening means comprises a metal strip. 20.The electronic system of claim 16 wherein the stiffening means comprisescopper.
 21. A method for preventing damage to metal conductors on or ina flexible circuit, the method compromising providing at least onestiffener at either one or both of a region where the flexible circuitoverlaps a panel edge, and a region where the flexible circuit is bentwherein the at least one stiffener is co-planar with traces of theflexible circuit and wherein the at least one stiffener is positionedalong a central portion of the flexible circuit and parallel to an edgeof the flexible circuit.
 22. The method of claim 21 comprising formingthe stiffener by retaining metallic strips during an etching processingduring fabrication of the flexible circuit.
 23. The method of claim 21comprising placing notches on the flexible circuit in a region where theflexible circuit is bent.
 24. An electronic system comprising: at leastone panel; at least one flexible circuit connected to the panel; theflexible circuit comprising at least one stiffener, wherein thestiffener is co-planar with traces of the flexible circuit, wherein theat least one stiffener is connected to at least one of a power source, asignal source, and a test trace, and wherein the at least one stiffenerresides in either one or both of: a region where the flexible circuitcrosses an edge of the panel such that the stiffener extends across theedge; and a region where the flexible circuit bends such that thestiffener extends around the bend.
 25. A electronic system comprising:at least one panel; at least one flexible circuit; and means forstiffening a portion of the flexible circuit wherein the means forstiffening is co-planar with traces of the at least one flexible circuitand wherein the means for stiffening is connected to at least one of apower source, a signal source, and a test trace.
 26. A method forpreventing damage to metal conductors on or in a flexible circuit, themethod compromising providing at least one stiffener at either one orboth of a region where the flexible circuit overlaps a panel edge, and aregion where the flexible circuit is bent wherein the at least onestiffener is co-planar with traces of the flexible circuit and whereinthe at least one stiffener is connected to at least one of a powersource, a signal source, and a test trace.